Porous ceramic foam materials are known in the art, for example, having been described in U.S. Pat. Nos. 3,090,094 and 3,097,930. These porous ceramic foam materials are known to be particularly useful in filtering molten metal, as described in copending U.S. Pat. Application SN 430,229 for "Improved Molten Metal Filter" by Michael J. Pryor and Thomas J. Gray, filed Jan. 2, 1974, now U.S. Pat. No. 3,893,917.
Molten metal, particularly molten aluminum, in practice generally contains entrained solids which are deleterious to the final cast metal product. These entrained solids appear as inclusions in the final cast product after the molten metal is solidified and cause the final product to be less ductile or to have poor bright finishing and anodizing characteristics. The inclusions may originate from several sources. For example, the inclusions may originate from surface oxide films which become broken up and are entrained in the resultant molten metal. In addition, the inclusions may originate as insoluble impurities, such as carbides, borides and others or eroded furnace and trough refractories.
It is naturally highly desirable to devise an improved filter for use in removing or minimizing entrained solids from the final cast product, particularly with respect to molten aluminum and especially, for example, when the resultant metal is to be used in a decorative product, such as decorative trim or sheet made from the 5000 series of aluminum alloys, as aluminum Alloys 5252 and 5657. Other aluminum alloys which benefit from improved filtration include: aluminum capacitor foil made from the 1000 series of aluminum alloys, as aluminum Alloys 1145 and 1188 in order to reduce pinhole defects in light gage products and maximize productivity in rolling; high strength extrusion alloys, such as aluminum Alloys 2024 and 7075 in order to obtain high ultrasonic quality; and extrusion alloys of the 6000 series of aluminum alloys, as aluminum Alloy 6061, in order to obtain higher productivity in extrusion operations; etc.
Inclusions as aforesaid cause loss of properties in the finally solidified alloy and lead to degradation of processing efficiency and loss of properties in the final product. For example, one type of finishing flaw which is particularly significant in decorative trim or sheet made from aluminum Alloy 5252 is a stringer defect known as a linear defect.
Rigorous melt treatment processes such as gas fluxing minimize the occurrence of such defects; however, these are not successful in reducing them to a satisfactory level for critical applications. Conventionally, melt filtration is utilized in order to decrease the extent of such defects, and others caused by the presence of inclusions in the melt. The most common form of melt filtration involves the use of open weave glass cloth screens placed in transfer and pouring troughs or in the molten pool of metal in the top of a solidifying ingot. Such filters have been found to be only partially effective since they remove only the larger inclusions. Another type of filter in common use is a bed filter made up, for example, of tabular alumina. Such filters have many disadvantages, perhaps the most serious of which is the great difficulty experienced in controlling and maintaining the pore size necessary for efficient filtration. Another difficulty with such filters is their tendency to produce an initial quantity of metal having poor quality at the start up of each successive casting run. This behavior results in a so-called ingot "butt effect," that is, ingots having butt portions of relatively poor quality which must be scrapped and recycled. Still further, the metal in a bed filter must be maintained molten even when the filter is not in use.
Accordingly, it is a principal object of the present invention to devise an improved filter which is particularly useful for filtering molten metal, and a method for filtering molten metal.
It is a further object of the present invention to devise a ceramic foam filter which is highly advantageous in filtering molten metal, especially molten aluminum, and which is convenient to prepare at a reasonable cost.
It is a still further object of the present invention to provide an improved filter and method as aforesaid which overcomes the art disadvantages referred to above, does not contaminate the melt and does not result in degradation of desirable characteristics in the final metal product.
Further objects and advantages of the present invention will appear hereinafter.